What is LP?
Limited Power (LP) is an optional designation developed because of concerns about excessive heat rise with power levels in excess of 60W. In testing that Berk-Tek and others have done representing high power applications, heat rise can reach levels that exceed a cables temperature listing. As a result, United Laboratories (UL) created a new optional marking of LP. This optional marking indicates that the cable has been evaluated by UL to carry the designated current in a reasonable worst-case installation without exceeding the temperature rating of the cable.
The LP test shown in Figure 1 includes a 192 homogenous cable bundle placed in a six (6) foot commercially available non-metallic conduit sized to the minimum diameter needed to accommodate the bundle. The ends of the conduit are stuffed with insulation to eliminate any air flow through the conduit. Each conductor of every cable in the bundle is then energized with a predetermined current ranging from 0.5A to a maximum of 1.0A in 0.1 increments and the temperature rise is measured with three temperature probes placed inside the bundle.
As an example, a cable under test is bundled and placed in the conduit as described above. A current of 0.5A per conductor, representing the approximate draw of a 100W device, is applied and the thermal rise of the cable is measured. After the maximum temperature rise is recorded, the results must meet the following criteria: Temp Rise from Electric Current + 45˚C (maximum assumed ambient environmental condition) ≤ Cable’s maximum rated temperature listing. If the cable in the example had a 20˚C heat rise and had a temperature listing of 75˚C, then: 20˚C + 45˚C ≤ 75˚C. Therefore, the cable would be given the optional LP designation of LP (0.5).
LP and the 2017 National Electrical Code (NEC)
Within Article 840, the 2017 NEC added a new Part VI Premise Powering of Communications Equipment over Communications Cables and within, Article 840.160 Powering Circuits the code delineates a power threshold of 60 W. When that threshold is exceeded the reader is redirected to Article 725.144 Transmission of power and Data for instructions on the allowed implementation of standard Class 2 and Class 3 cabling as well as newly added LP marked cables. It is important to note that the LP indicator is not a safety listing, rather it is an optional marking representing specialized cabling that is permitted to be installed in the same fashion as traditional CL2 and CL3 cables. LP cables are not required for PoE type power distribution; rather they are an additional permitted construction. Also included in Article 751.144 is an ampacity Table 725.144 where the ampacities are listed for various bundle sizes depending on numerous factors like the cables AWG size and temperature listing. Note 1 on the bottom of table 725.11 states; “For bundle sizes over 192 cables, or for conductor sizes smaller than 26AWG, ampacities shall be permitted to be determined by qualified personnel under engineering supervision.” This is exactly why Berk-Tek developed our PoE icons for our products indicating the maximum recommended bundle size with each cable energized to 100W regardless of the optional LP designation.